The primary objective of this study is to compare a set of diagnostic tests commonly used to identify patients with somatotropin deficiency and determine the relative utility of each test for diagnostic use in somatotropin-deficient adult populations. Using the null hypothesis, we expect to find no difference between the tests in ability to discriminate clinically diagnosed somatotropin-deficient individuals from healthy control subjects when matched for age, gender, ethnic origin, and body mass index. The secondary objective of this study is to evaluate the value of these stimulation tests as well as that of plasma IGF-I levels in a sub-population of patients who have a high potential for somatotropin deficiency. A third objective of the study is to evaluate the safety of each test by determining and recording the frequency and severity of adverse events associated with each test. This will be a multi-center study of cross-sectional design, involving 100 patients. Approximately 20 to 25 patients will be studied at our institution. Three patients' populations will be represented in this study. The somatotropin-deficient (diseased) population will be represented by approximately 40 patients who have been diagnosed to have adult-onset hypopituitarism with multiple hormone deficiencies (two or more previously identified hormone deficiencies). The control population will be represented by approximately 40 healthy volunteers between 18 and 70 years of age. Volunteer selection will be matched so that the group composition resembles that of the multiple hormone- deficient experimental group for the following variables: age, gender, ethnic origin, and body mass index (BMI). A third group will include approximately 20 patients who have been clinically diagnosed as having organic pituitary disease and either one or no known hormonal deficiencies. The diagnostic utility of five somatotropin stimulation tests and one somatotropin-dependent biochemicla marker, (IGF-1) will be evaluated. Each test will be judged according to its agreement with the clinical diagnosis of the patient (either somatotropin deficient or normal). Stimulation tests include the following: Arginine; l-dopa; arginine/ l-dopa; arginine/growth hormone-releasing hormone (GHRH) and insulin tolerance test (ITT). The order of stimulation test administration will be determined by a pre-written randomized table listing a specific test sequence for each patient number.